Preparation and use of membranes for separation of solids, liquids, and gases at molecular size levels through ultrafine pores in the membrane or by selective permeability of the membranes for the solids, liquids, and gases, is well known. These membranes are usually very thin and mechanically weak so that to facilitate their handling and use, the practice of combining them with stronger membranes or films for support has been widely adopted.
Among the more desirable materials for use as supports are porous fluoropolymer membranes. Fluoropolymers, and particularly polytetrafluoroethylene (PTFE), are desirable because of their high heat resistance and high chemical resistance. However, these membranes are hydrophobic and cannot be used as supports where hydrophilicity is required. Also, their non-stick properties make permanent attachment to other materials very difficult.
In order to provide such hydrophobic porous fluoropolymer membranes with a capability of permeating water therethrough, it is necessary to make the inner surfaces of the fine pores of the fluoropolymer membranes hydrophilic. Review has been made of various methods for making the inner surfaces of fine pores of porous fluoropolymer membranes hydrophilic. Such methods may include, for example, (1) the method of replacement with water after impregnation of the membrane with a hydrophilic organic solvent such as an alcohol; (2) the method of impregnating the membrane with a surfactant through its alcohol solution; (3) the method of impregnating the membrane with a monomer containing a hydrophilic group and then polymerizing the monomer; (4) the method of making the membrane hydrophilic by impregnation with a hydrophilic polymer such as polyvinyl alcohol through its aqueous solvent solution; (5) the method of impregnating the membrane with a monomer containing a hydrophilic group and then graft-polymerizing the monomer by treatment with strong reducing agents or plasma gases, or by irradiation with high energy radiation such as gamma rays, electron beam, etc.
These conventional methods, however, have various drawbacks. Method (1) has the problem that the effect is lost once the materials have been dried. Method (2) has the drawback that the surfactant may likely elute thereby worsening durability. Methods (3) and (4) may cause clogging of the fine pores of the membrane and may cause low durability as the polymer may be likely to elute. Consequently, cross-linking of the impregnated polymer are sometimes carried out for the purpose of preventing elution of the polymer in the methods (2), (3) and (4). These treatments are described in examined Japanese Patent Publication Nos. 21270/1978, 8669/1979, 154737/1981 and 98640/1989. Method (5) may deteriorate the mechanical properties of the materials and furthermore it may be difficult to prevent homopolymerization of the monomer and the elution of the homopolymer.